FIG. 9 shows a configuration of a prior-art smelting reduction furnace.
The smelting reduction furnace is provided with a reaction vessel 50, and molten slag Sa is formed in this reaction vessel 50. In a roof section of the reaction vessel 50, a raw-material charging port 51 and a gas exhaust port 52 for exhausting a gas generated in the reaction vessel 50 are provided. This gas exhaust port 52 is connected to an exhaust gas treatment device or the like through a duct (See Patent Document 1, for example).
In an upper part of the reaction vessel 50, a plurality of upper tuyeres 53 for blowing oxygen are provided for secondary combustion of the gas generated in the furnace while in a lower part, lower tuyeres 54 for agitating the molten slag Sa by introducing oxygen (or oxygen-enriched air) are provided.
On the other hand, on both outsides of the lower part in the reaction vessel 50, a molten iron pool 56 communicating with the reaction vessel 50 through one siphon 55 and a slag pool 58 communicating with the reaction vessel 50 through the other siphon 57 are provided, respectively. In the molten iron pool 56, an iron outlet 59 for discharging molten iron M is provided, while in the slag pool 58, a slag outlet 60 for discharging slag S is provided.
In the smelting reduction furnace having the above configuration, iron materials, carbonaceous materials such as coal, coke and the like, and raw materials such as a slag forming material are continuously charged into the reaction vessel 50 through the raw-material charging port 51. As a result, the materials are melted and the molten slag Sa are produced in the reaction vessel 50. By blowing oxygen through the lower tuyeres 54 to this molten slag Sa, the molten slag Sa is agitated.
The materials having been charged into the molten slag Sa are mixed with the molten slag Sa so as to become molten slag, and iron oxide in the slag is reduced by charged coal.
CO gas and the like generated in the molten slag Sa reacts with the oxygen blown into a secondary combustion zone, and then is oxidized. The gas generated in the reaction vessel 50 is led to the outside of the furnace through the gas exhaust port 52 and discharged to the outside of the system through a flue gas treatment device.
In the lower part in the reaction vessel 50, molten iron (or pig iron) and the molten slag S are separated because of a difference in each specific gravity.
In the above-mentioned prior-art melting furnace, carbon in the molten iron/slag and oxygen react, and then a CO gas is generated, which causes air bubbles to be generated. Moreover, viscosity increases as content of iron oxide in the slag increases, and the generated CO gas tends to be contained and retained in the slag. As the number of air bubbles is increased in the slug, the slag becomes foamy, that is, so-called slag foaming tends to occur.
If excess slag foaming is left as it is, that causes trouble that high-temperature slag is ejected to the outside of the furnace through seal portions of a furnace top, a furnace cover and the like, and productivity is decreased. The slag foaming indicates unbalance of a reaction between oxidization and reduction in the furnace, which more or less gives a bad influence to a production process. Thus, in order to continuously produce molten metal stably, the slag foaming needs to be suppressed.
As means for suppression, a suppressor (the carbonaceous material, for example) for suppressing the slag foaming can be added to the slag, but appropriate addition is difficult due to several reasons. Specifically, in a process of continuously producing the molten metal, fluctuation in chemical components of the materials and amounts of charged/discharged materials cannot be controlled to zero. In addition, unexpected reactions occur in the furnace. Moreover, since temperature in the inside of the furnace is too high and amounts of generated molten dust and fume are large, it is practically impossible to monitor the inside of the furnace throughout the blowing time.    Patent Document 1: Japanese Unexamined Patent Application Publication No. 11-310814